The present invention relates to semiconductor device fabrication and integrated circuits and, more specifically, to semiconductor structures and methods of forming semiconductor structures.
Devices fabricated using silicon-on-insulator technologies may exhibit certain performance improvements in comparison with comparable devices built directly in a bulk silicon substrate. Generally, a silicon-on-insulator wafer includes a thin device layer composed of a semiconductor material, a substrate, and a buried oxide layer physically separating and electrically isolating the device layer from the substrate.
Device structures for a field-effect transistor generally include a source, a drain, and a gate electrode configured to switch carrier flow in a channel region arranged between the source and drain. The channel region of a planar field-effect transistor is located beneath the top surface of a substrate on which the gate electrode is arranged. When a control voltage exceeding a designated threshold voltage is applied to the gate electrode, carrier flow occurs in the channel region to produce a device output current.
Field-effect transistors fabricated by complementary metal-oxide semiconductor processes also include n-type and p-type wells that are formed by ion implantation in the substrate. A BFMOAT is another type of region that may be formed in the substrate and from which the implants forming the wells are blocked. The substrate region retains its original conductivity type (e.g., lightly-doped p-type silicon) over the BFMOAT and has a high electrical resistivity in comparison with the semiconductor material of the wells. During device operation, the BFMOAT may operate to minimize substrate coupling and/or to improve the signal-to-noise ratio. However, during high current drive operation, the device output current may drop due to charge accumulation in the substrate at and near the interface with the buried oxide layer.
Improved semiconductor structures and methods of forming semiconductor structures are needed.